Battery level indicator

Hi makers!

I would like to show you a circuit, 10-LED battery charge level indicator centered around the LM3914 (U1) linear display driver integrated circuit. It provides a visual representation of a battery's voltage using an LED bar graph.

Here is a detailed breakdown of how each section of the circuit functions:

1. Power Supply & Switching

• J1 (Battery Input): This header connects the battery to the circuit. Pin 1 is the positive terminal (VCC), and Pin 2 is the negative/ground terminal (GND).
• J3 (On/Off Switch): A single-pole, double-throw (SPDT) switch configuration. When closed (shorting pins 2 and 3), it delivers power from the battery to the main (VCC) rail of the circuit, which powers the LM3914 (Pin 3) and supplies the common anodes of the LED display.

1. Voltage Reference & Scaling (Calibration)

The LM3914 uses an internal 1.25V reference to establish the upper and lower thresholds of its internal resistor ladder:

• High Reference (V_RHi): Pin 7 (Out) and Pin 6 (Rhi) are tied together, setting the top of the measurement scale to the chip's internal 1.25V reference. Pin 8 (ADj) is tied to ground, keeping this reference stable against the ground rail.
• Low Reference (V_RLo): Pin 4 (Rlo) determines the voltage level at which the lowest LED (first indicator) turns on. It is connected to potentiometer RV1 (5K), which allows the user to adjust and calibrate the exact "low battery" threshold.

1. Input Signal Sensing

• Pin 5 (Sig): This is the analog input pin that samples the voltage to be measured.

• RV2 (50K) & R1 (3K):

  Because a battery’s voltage is typically much higher than the chip's internal 1.25V reference limit, RV2 and R1 form an adjustable voltage divider. This network scales down the full battery voltage so that it falls precisely within the V_RLO to V_RHI window. RV2 is adjusted during calibration to map the "full battery" voltage to the highest LED.

1. Display & Mode Selection

• BAR1 (HDSP-4830_2): A 10-segment LED bar graph display. The LM3914 features internal constant-current sinks on its output pins (Pins 1 and 10–18), meaning no current-limiting resistors are required in series with individual LEDs.
• J2 (Selector / Mode Switch): Controls how the LEDs display the information:
  • Bar Mode: When a jumper connects pins 2 and 3, Pin 9 (Mode) is tied to (VCC). In this mode, the display fills up like a solid bar (e.g., at 50% charge, LEDs 1 through 5 light up).
  • Dot Mode: When J2 is left open, Pin 9 is disconnected. In this mode, only a single LED lights up to represent the current voltage level, which significantly reduces power consumption.

This circuit description was created using AI tool.

Now, let me show you another interesting aspect: I built a circuit like this at home. It took quite a bit of work—I even made a double-layer PCB. Was it worth it? Well, for the time being, yes.

I just designed the PCB for my own use, I am not the original author. This project is presented ""as is"" and is intended for personal use.

Before sending a PCB off for manufacturing, I prefer to experiment using either a breadboard or a test board. In this case, however, experimenting on a breadboard is easy. After that, an order could be placed with JLCPCB.

#PCB #LM3914 #LM3915